It is known that a MTJ (Magnetic Tunnel Junction) element serving as a magnetoresistive element has a stacked structure as a basic structure, and shows a tunneling magnetoresistive (TMR) effect. The stacked structure is formed by a first ferromagnetic layer, a tunnel barrier layer, and a second ferromagnetic layer. Such MTJ elements are used in 100-Mbpsi (bits per square inch) HDD heads and magnetic random access memories (MRAMs).
A MRAM characteristically stores information (“1”, “0”) corresponding to changes in the relative angle of the magnetizations of magnetic layers in each MTJ element, and therefore, is nonvolatile. A magnetization switching speed is several nanoseconds, and accordingly, high-speed data writing and reading can be performed. In view of this, MRAMs are expected to be the next generation of high-speed nonvolatile memories. Where a spin torque transfer switching technique for controlling magnetization through a spin-polarized current is utilized, the current density becomes higher when the cell size of the MRAM is made smaller. Accordingly, a high-density, low-power-consumption MRAM that can readily invert the magnetization of a magnetic material can be formed.
Furthermore, it has been recently theoretically proved that the use of MgO for the tunnel barrier layer leads to a magnetoresistance ratio of as high as 1000%, which has been drawing attention. Specifically, by crystallizing MgO, only the electrons having a certain wavenumber can selectively tunnel from the ferromagnetic layers while maintaining the wavenumber. At this point, the spin polarizability has a large value in a certain crystal orientation, and as a result, a giant magnetoresistive effect is caused. Therefore, an increase in the magnetoresistive effect of each MTJ element leads directly to a higher density and lower power consumption of the MRAM.
To achieve a higher-density nonvolatile memory, higher integration of magnetoresistive elements is essential. In the ferromagnetic materials forming the magnetoresistive elements, however, thermal disturbance resistance characteristics become lower as the element size becomes smaller. Therefore, how to increase the magnetic anisotropy and thermal disturbance resistance characteristics of a ferromagnetic material remains a challenge.
To solve the above problem, test MRAMs have been produced recently with the use of perpendicular magnetization MTJ elements in which the magnetization direction of each ferromagnetic material is perpendicular to the film plane. In a perpendicular magnetization MTJ element, a material with large crystalline magnetic anisotropy is normally used for a ferromagnetic material. Such a material has magnetization oriented in a certain crystalline direction, and the size of the crystal magnetic anisotropy can be controlled by varying composition ratio and crystallinity of the constituent element. Therefore, the magnetization direction can be controlled by switching the crystal growth direction. As a ferromagnetic material has high crystal magnetic anisotropy, the aspect ratio of the element can be set at 1. Further, having high thermal disturbance resistance characteristics, such a material is suitable for high integration. In view of this, forming perpendicular magnetization MTJ elements each having a great magnetoresistive effect is essential in realizing a high-integration, low-power-consumption MRAM.